The present invention relates generally to communications systems and, more particularly, to discrete multi-tone communications systems and digital subscriber line communication systems.
Remote client personal computers (PCs) are typically connected to the Internet over a telephone network of twisted-pair transmission lines via analog modems, as illustrated in FIG. 1. Unfortunately, analog modem speeds are currently limited to 56 thousand bits per second (56 Kbps) because of various constraints of existing telephone networks.
Asymmetrical Digital Subscriber Line (ADSL) technology is currently being developed to extend bandwidth over twisted-pair transmission lines by an order of magnitude. ADSL is based on a discrete multi-tone (DMT) transmission system. ADSL transmission rates over twisted pair transmission lines can reach as high as 8 million bits per second (8 Mbps) downstream (i.e., towards a client PC) and up to 256 Kbps upstream (towards a server). Data transmission rates for ADSL are substantially higher in the downstream direction than in the upstream direction. The asymmetric nature of ADSL fits well with data delivery requirements of the Internet. More data is typically transmitted downstream to a client PC than upstream from a client PC to a server.
Unfortunately, a significant problem currently plaguing the implementation of ADSL is the bandwidth burden placed on a telephone network by ADSL as illustrated in FIG. 2. In FIG. 2, lines 20 correspond to individual analog lines, such as local subscriber loops or lines and plain old telephone system (POTS) connections to a telephone company central office (CO) switch. Lines 22 correspond to T-1 lines (i.e., lines having bandwidth capabilities of 1.544 Mbps and capable of carrying 24 digitized voiceband connections). Lines 24 correspond to T-3 lines-(i.e., lines having bandwidth capabilities of 28 T-1 lines and that are time-division multiplexed).
Although 24 analog POTS connections can be handled by a single T-1 line, 24 ADSL connections utilize almost the entire bandwidth of a T-3 line. This bandwidth constraint is further complicated by the fact that many analog subscriber lines may not terminate at a CO switch. Analog subscriber lines may terminate in remote locations, such as a manhole or utility box in a neighborhood, where a subscriber line is digitized and then routed to a CO switch via a T-1 or T-3 line. Increasing bandwidth for these remote links may be costly and may complicate implementation of ADSL.
In addition, current ADSL implementations typically include a dedicated pair of ADSL modems for each subscriber line. This is in contrast to voice-band modems, where a server only utilizes dedicated modems for each active connection. Requiring a dedicated pair of ADSL modems for each subscriber line may add significantly to the cost of ADSL implementation by telephone service providers.
Various solutions have been proposed to alleviate bandwidth constraints and thereby facilitate implementation of ADSL technology. One solution involves statistical multiplexing at the remote site. Another solution proposes terminating multiple ADSL subscriber lines in a single shared modem at the client side. This may simplify hardware requirements on the telephone network side of each ADSL subscriber line because each subscriber line may share a high-speed transmitter. A single shared modem eliminates the need for separate ADSL modems for respective ADSL subscriber lines.
Unfortunately, a limitation of a single shared ADSL modem is that modems on the other end of subscriber lines connected to the shared modem may need to receive the same signal. Accordingly, the bandwidth of the transmitted signal within each frequency band may need to be reduced to accommodate the subscriber line in the group with the lowest bandwidth within each of these frequency bands. Accordingly, the net bandwidth that can be exploited by a shared modem on the server end may be less than that of a single modem on the worst line.
Various ways of overcoming limitations with shared ADSL modems have been proposed. For example, U.S. Pat. No. 5,557,612 to Bingham relates to a system for establishing communications between a central unit and a plurality of remote units wherein access requests include an identification of each remote unit. A particular subchannel is allocated to the remote units by the central unit in an ADSL application. Units requiring bandwidth already allocated to other clients may be denied service. U.S. Pat. No. 5,608,725 to Grube et al. relates to establishing a communications system wherein a primary site is coupled to a plurality of secondary sites via low pass transmission paths. However, only a single secondary site may be active at a time. Furthermore, the primary site allocates carrier channels of the inbound and outbound low pass transmission paths.
In view of the above discussion, it is an object of the present invention to facilitate the implementation of ADSL by including statistical multiplexing within an ADSL modem.
It is another object of the present invention to facilitate the implementation of ADSL without requiring a separate ADSL modem at each end of a subscriber loop.
It is another object of the present invention to help lower costs associated with ADSL implementation.
These and other objects of the present invention are provided by a multi-drop Asymmetrical Digital Subscriber Loop (ADSL) modem having at least two sets of digital front end circuitry which allow for simultaneous connections to at least two local loops. A shared DSL modem is operably associated with each of the two or more sets of digital front end circuitry and transmits and receives data to and from the digital front end circuitry. A digital interface is operably associated with the shared DSL modem and is configured to communicate with an external network.
By providing multiple analog front ends and shared modem components, the present invention allows for multiple ADSL local loops to be provided by a single modem. The shared components reduces the cost of each connection by reducing the number of components required for the modem. Furthermore, space constraints may be overcome by allowing multiple local loops to connect to a single modem. Accordingly, the total number of modems required and the total space and power consumption needed to support the modems may be reduced by the multi-drop ADSL modem of the present invention.
The digital front end circuitry may include a transmit isolation buffer associated with each local loop. The transmit isolation buffer isolates transmitted communication signals of the local loops from each other and is operably associated with a transmitter portion of the shared DSL modem. The digital front end circuitry may also include a receive isolation buffer associated with each local loop which isolates received communication signals of the local loops from each other and which is operably associated with a receiver of the shared DSL modem.
The digital front end circuitry may also include a receive filter and a sample and hold circuit associated with each receive filter. The receive filter is operably associated with the receive isolation buffer for filtering signals received by the analog front end circuitry. The sample and hold circuit associated with each receive filter receives filtered signals from the corresponding receive filter. An analog to digital converter may be associated with each sample and hold circuit to receive a held signal from the corresponding sample and hold circuit and convert the analog signal to a digital signal.
The shared ADSL modem may include a receiver having a multiplexer and an analog-to-digital converter. The multiplexer receives an output of the receive filter of each local loop and provides one of the outputs of one of the receive filters as a multiplexer output. The analog-to-digital converter receives the multiplexer output and converts the multiplexer output to a digital signal. The receiver may also include a timing equalizer and a memory device. The timing equalizer utilizes local loop specific equalizer information to equalize the digital signal from the analog-to-digital converter and to time-domain equalize the digital signal to provide an equalized digital signal. The memory device is operably associated with the timing equalizer and stores local loop specific equalizer coefficients.
The receiver may further include a serial to parallel converter, a Discrete Fourier Transform circuit, a symbol detector, and a memory device operably associated with the symbol detector. The serial to parallel converter converts the equalized digital signal from a serial signal to a parallel signal so as to provide a parallel digital signal. The Discrete Fourier Transform circuit transforms the parallel digital signal from a time domain signal to a frequency domain signal so as to provide digital frequency coefficients. The symbol detector receives the digital frequency coefficients and detects symbols from the frequency domain coefficients based on local loop specific symbol information to provide client specific symbols. The memory device operably associated with the symbol detector stores local loop specific symbol information.
The receiver may also include a decoder and a memory device operably associated with the decoder. The decoder receives client specific symbols from the symbol detector, descrambles, provides Reed-Solomon decoding and de-interleaves the symbols based on client specific information to provide client specific data. The memory device operably associated with the decoder stores the client specific information.
The ADSL modem may also include a symbol buffer and a packet buffer for each local loop. The symbol buffer receives symbols from the symbol detector and provides symbols to the decoder. The packet buffer receives packets from the decoder.
The ADSL modem may also include a transmitter having an encoder, a memory device operably associated with the encoder, a symbol mapper, and a memory device operably associated with the symbol mapper. The encoder receives client specific symbols for transmission on a local loop and scrambles, provides Reed-Solomon encoding and interleaves the symbols based on client specific information to provide client specific symbol data. The memory device operably associated with the encoder stores the client specific information. The symbol mapper receives client specific symbol data and maps the client specific symbol data to frequency domain symbols based on local loop specific frequency information to provide local loop specific symbol data. The memory device operably associated with the symbol mapper stores local loop specific frequency information.
The transmitter may also include an Inverse Discrete Fourier Transform circuit, a parallel to serial converter, a digital to analog converter, and a transmit filter. The Inverse Discrete Fourier Transform circuit transforms the local loop specific symbol data from frequency domain data to a time domain signal so as to provide parallel digital symbol data. The parallel to serial converter converts the parallel digital symbol data from a parallel signal to a serial signal so as to provide a serial digital signal. The circuit also adds a framing bit and a cyclic prefix. The digital to analog converter converts the serial digital signal to an analog signal. The transmit filter provides a filtered transmit signal to the transmit isolation buffers.
According to another aspect of the present invention, methods and computer program products are provided for allowing multiple simultaneous local loops to connect to a single ADSL modem. Operations include isolating the local loops from one another, storing loop dependent information for each connected local loop, and communicating with each local loop utilizing the loop dependent information.